High frequency system



0a. 21, 1941. A, WIESSNER 2,260,269

HIGH FREQUENCY SYSTEM Filed March 4, 1939 (OMPOS/TE Fun-a6 we Pas-J 7'0 l? mm 4014 69w: I $3 0 /n venfvr: Affred M'essnef trorngy,

Patented Oct. 21, 1941 HIGH FREQUENCY SYSTEM Alfred Wiessner, Berlin, Germany, assignor to C. Lorenz Aktiengesellschaft, Berlin-Tempelhof,

Germany, a company Application March 4, 1939, Serial No. 259,727 In Germany March 5, 1938 1 Claim.

This invention relates to high frequency sys-- tems, and more particularly to improvements in or relating to means for coupling high frequency receiving devices to lines conducting high frequency and low frequency message currents.

The wire-radio intelligence transmission or broadcasting over telephone lines by means of high frequency carrier currents is generally accomplished in such manner that a plurality of speech or tone frequency modulated high frequency carrier waves are impressed upon the subscribers lines, e. g. in central exchanges. This method is not only applicable for transmitting or broadcasting entertainments, but also for transmitting alarm messages and the like.

It is highly desirable in such systems to employ receiving devices which are likewise capable of picking up wireless broadcasting performances, but experience teaches that the selectivity of the conventional single circuit or primary receivers is insufficient in many cases. For instance, when the frequency difference between two transmitters is less than 30 cycles per second, it becomes impossible to reliably distinguish either of these adjacent transmitters. In order to overcome this disadvantage, it has been suggested to have such receivers preceded by an apparatus which effects a first selective action in order to insure an increased selectivity of the receiver connected thereto, but it is a matter of fact that such apparatus introduces considerable difiiculties when connected to an incoming line, since the almost inevitable inaccuracies with respect to the tuning thereof detrimentally influence the wire radio transmission lines.

The main object of this my invention is to overcome these difliculties in a simple and effective manner.

According to the main feature of my invention, the incoming voltages at the receiving end of the wire radio transmission line are decreased to such an extent by means of a voltage divider that no reception is possible in the receiver unless this apparatus is rendered reactive by correspondingly adjusting the feedback thereof. This expeclient so increases the selectivity of the receiver, even if simple conventional single circuit receivers are employed that transmitters operating on adjacent frequencies of the frequency spectrum will be readily distinguished from one another, provided that the frequency difference between the transmitters is not too small.

In cases that attempts would be made to overcome the heretofore mentioned difficulties in a ill manner different from that proposed in accordance with my present invention, e. g. by decreasing the signal amplitudes in the transmitter position so as to directly permit signal currents of low amplitude to control the receivers, the ratio of the signal amplitude to the inevitable noise amplitude would become shifted in an unfavourable sense, while the adequate ratio is sustained by the use of a voltage divider as proposed in the invention without running the risk that too large signal amplitudes would upset the possibilities for reliably selecting the desired transmitter in the receiving equipment.

The accompanying drawing diagrammatically shows one embodiment of the present invention. An incoming high frequency message transmission line over which likewise low frequency messages or other high frequency messages are propagated is connected to a composite filter device having a low pass section W and a high pass section V to the telephone T and a high pass filter V. The output side of the high-pass filter is connected to a voltage divider S from which a partial voltage may be derived and applied to the high frequency receiver R. The amplitude of this partial voltage is so chosen that the feedback circuit of the receiver must be rendered reactive in order to enable reception. This compulsorily required regenerative action in turn enhances the selectivity of the receiver.

A further advantage obtained by the above described arrangement consists in the fact that parasitic oscillations occurring in the telephone set T which is likewise connected to the filter device W do not harmfully influence the receiver in any way since the ratio of the amplitudes of such oscillations to the effective amplitudes at the voltage divider is correspondingly lowered.

What is claimed is:

In combination, a transmission line for transmitting a band of carrier frequencies and low frequency signalling current, a composite filter having high pass and low pass sections terminating said line for separating a band of carrier frequencies from the low frequency signalling current, a voltage divider connected to the output of the high pass filter section of said filter, and a regenerative receiver connected across a portion of said voltage divider, the connection points of said regenerative receiver being such that the output signals from said receiver could not be perceived without the regenerative action, whereby selective reception is increased.

ALFRED WIESSNER. 

